1. Field of the Invention
The present invention generally relates to a semiconductor memory device, and more particularly, to a semiconductor memory device improved such that it has increased capacitance of a capacitor. The present invention also relates to a method of manufacturing such a semiconductor memory device.
2. Description of the Background Art
Three dimensional structure is essential to the capacitor of Dynamic Random Access Memory (DRAM) due to the reduction of its cell size. Various capacitor structures have been proposed. For example, the capacitor structures such as fin type and crown type are known. One proposed method of increasing the surface area of a capacitor electrode (storage node electrode) formed of polysilicon is to form concave and convex parts on the surface. The polysilicon thus obtained is called rugged polysilicon for its surface structure.
A method of manufacturing a semiconductor memory device having a capacitor of the rugged polysilicon is disclosed, for example, in Applied Physics, Vol. 61, No. 11, p.p. 1147-1151. The method will be hereinafter described with reference to appended figures.
Referring to FIG. 10, an interlayer insulating film 2, is formed on a silicon substrate 1. A contact hole 2a for exposing the surface of silicon substrate 1 is provided in interlayer insulating film 2.
Referring to FIG. 11, an amorphous silicon 3 doped with phosphorus is formed on silicon substrate 1 such that it fills contact hole 2a. Doping of phosphorus is for the purpose of making amorphous silicon 3 conductive.
With reference to FIGS. 11 and 12, doped amorphous silicon 3 is patterned to form a storage node electrode 4.
With reference to FIGS. 12 and 13, by annealing storage node 4 in disilane atmosphere, silicon crystal grains 4a are formed on its surface, producing storage node electrode 4 with concave and convex portions on its surface. Silicon crystal grains 4a are derived from amorphous silicon, grown into crystal by annealing. Although not shown in figures, a capacitor insulating film is then formed on the surface of storage node electrode 4, and a cell plate electrode is further formed thereon to complete a semiconductor memory device.
The outer surface area of the storage node electrode 4 is increased because concave and concave portions are formed thereon, resulting in an increased capacitance of the capacitor.
Using the conventional method of forming rugged polysilicon described above, however, a storage node electrode with sufficiently large surface area could not be obtained.